Conventionally, a halogen flame retardant has been widely used as a flame retardant to impart flame retardancy to a coating composition (for example, see Patent Literature 1). However, this method is not preferred in terms of environmental protection because it Poses a problem of dioxin and chlorofluorocarbon generated from the halogenated flame retardant. Further, inorganic flame retardants such as aluminum hydroxide are also used as a flame retardant to impart flame retardancy to a coating composition. However, aluminum hydroxide has a problem of reducing the physical properties of a coating material and a substrate to which the coating material is applied and physical properties such as water resistance.
There is also a method of coating a plastic with an inorganic coating material to impart flame retardancy to the plastic. However, since the inorganic coating material has a low flexibility and is easily cracked, it is difficult to form a thick coating film. Thus, the coating material itself has flame retardancy, but in the case of a composite with a plastic, the heat from flame will be transferred to the plastics when the flame is brought into contact with the composite, and the plastic will carbonize or result in spread of fire. Further, in the case of a coating material containing an organic binder, the organic binder will carbonize when the flame is brought into contact with the coating material. Therefore, a plastic coated with these coating materials is difficult for use in the applications in which non-carbonizing properties are required such as a railway rolling stock application.
Conventionally, there has been known an illumination cover for railway rolling stocks prepared by applying an inorganic coating agent to a glass fiber sheet. However, a light diffusing member prepared by coating or impregnating a glass fiber sheet with a common inorganic coating agent has a problem of low flame resistance when a binder includes an organic component and has a disadvantage of low flexibility to be easily cracked when the binder is an inorganic component.
Patent Literature 2 discloses a sheet comprising at least one glass fiber fabric and a pair of resin layers sandwiching the glass fiber fabric, wherein the resin layer comprises a thermosetting resin such as vinyl ester or a photocurable resin. Further, Patent Literature 3 discloses a glass fiber sheet comprising at least one glass fiber fabric, a resin coating layer made of a thermosetting resin, wherein the glass fiber fabric is impregnated with the thermosetting resin followed by solidification of the resin to form the coating layer, and a bead layer on at least one surface of the resin coating layer. However, all of these light diffusing sheets have insufficient flame resistance and low flexibility.